Tendon Sheath

Tendon sheath is the name of the tubular double-layered sacs that surround some tendons.
(Tendons are tough pale/whitish cords that attach muscles to bones. Compare with ligaments, which attach bones to bones and are also inelastic, yet flexible.)

Tendon sheaths are tubular double-layered sacs lined with synovial membrane and containing synovial fluid.

Tendon sheaths enclose many tendons, including the flexor tendons of the wrist and ankle where they perform the important function of keeping friction as low as possible, while faciliating movement of the joint.

Disorder(s) of Tendon Sheaths:

Tenosynovitis is an inflammation of the tendon sheath. It can occur as a result of repeated or prolonged activities, such as forceful exertion, and local mechanical stress that cause the lubrication system of the tendon sheath to break down. This can lead to friction between the tendon and its sheath, causing inflammation and swelling of the tendon sheath. If this happens repeatedly then fibrous tissue may form, thickening the tendon sheath and hindering movement of the tendon.